Butterflies have a long tubular mouthpart (called a proboscis) that coils at the bottom of the head. The coiling is the resting position of the proboscis. The cuticle of the proboscis contains resilin, a rubbery protein known for its ability to spring back into position. Contraction of muscles will extend the proboscis. When the muscles relax, the proboscis naturally curls as the resilin springs back into its resting conformation.
Butterflies use the long tube to feed on nectar from flowers and to imbibe other liquids. These liquids can contain high concentrations of sugars and be quite viscous. These liquids travel up the proboscis and into the mouth. A set of muscles in the pharynx (a part of the food canal) contract and relax causing a change in pressure within the mouthparts. For many years, this process was thought to work the same way that we would suck liquids through a straw. When humans drink through a straw, we seal our lips around the straw and draw air out of our mouth. The drop in pressure causes the liquid to fill the straw and it moves upward into our mouths.
Recently, it has become clear that the butterfly cannot generate the necessary pressure differential to pump the viscous liquids into its mouth the same way we would drink through a straw. The food channel in the butterfly proboscis is far smaller than a typical drinking straw. It is not a tightly sealed cylinder as is a drinking straw. This has led scientists to reinvestigate how a butterfly is able to drink.
The butterfly proboscis is formed by the left and right halves of a set of mouthparts (the elongated galea of the maxillae) coming together to form a feeding tube. Ventrally (on the bottom) the left and right mouthparts are joined by tightly packed hooks that form a tightly sealed opening. Dorsally, the left and right mouthparts are joined by pairs of lance-shaped plates that overlap like a “zipper”. These plates move and are not tightly sealed. In the open conformation, liquids will flow in through the pores between the plates. Thus, butterflies can take up nectar through the top of their mouthparts as well as the tip.
In the closed conformation, liquids move up the narrow feeding tube in the direction of the reduced pressure (from the pumping in the pharynx) due to capillary action. As the butterfly pharynx pumps, the feeding tube alternately fills with liquid and air. The air bubbles, between the droplets of liquid are important in pushing the food up the feeding tube and into the mouth. A recent article in The Journal of the Royal Society , “Butterfly proboscis: combining a drinking straw with a nanosponge facilitated diversification of feeding habits” discusses butterfly feeding in more depth.
One of the Nikon Small World entries, “Tip of a Butterfly Tongue”, submitted by Stephen Nagy (below) clearly shows the zipper-like structure on the butterfly proboscis.